Streamlined Synaptic Vesicle Cycle in Cone Photoreceptor Terminals

Abstract

Cone photoreceptors tonically release neurotransmitter in the dark through a continuous cycle of exocytosis and endocytosis. Here, using the synaptic vesicle marker FM1-43, we elucidate specialized features of the vesicle cycle. Unlike retinal bipolar cell terminals, where stimulation triggers bulk membrane retrieval, cone terminals appear to exclusively endocytose small vesicles. These retain their integrity until exocytosis, without pooling their membranes in endosomes. Endocytosed vesicles rapidly disperse through the terminal and are reused with no apparent delay. Unlike other synapses where most vesicles are immobilized and held in reserve, only a small fraction (<15%) becomes immobilized in cones. Photobleaching experiments suggest that vesicles move by diffusion and not by molecular motors on the cytoskeleton and that vesicle movement is not rate limiting for release. The huge reservoir of vesicles that move rapidly throughout cone terminals and the lack of a reserve pool are unique features, providing cones with a steady supply for continuous release.

Calmodulin enhances ribbon replenishment and shapes filtering of synaptic transmission by cone photoreceptors

“…To explore whether the rate of vesicle collisions is itself rate limiting, we first assumed that all ribbon collisions result in successful attachment (s = 1), yielding a lower bound on the replenishment time constant,  a . Using empirically determined values from photoreceptor synapses of D = 0.11 µm 2 /s,  = 2,210 v/µm 3 , and  = 45 nm (Rea et al, 2004;Thoreson et al, 2004;Sheng et al, 2007), we obtained  a = 91 ms. This is roughly an order of magnitude faster than the experimentally measured value for  a .…”

Synaptic transmission at retinal ribbon synapses

“…Cone terminals of lizard and turtle retinas (~6 μm diameter) contain approximately 250,000 synaptic vesicles (Pierantoni and McCann, 1981;Rea et al, 2004). Assuming a similar cytoplasmic concentration of vesicles, a mammalian rod with a terminal 2-3 μm in diameter would have 8000-27,000 vesicles within the terminal cytoplasm.…”

“…For example, in Mb 1 terminals, the actin cytoskeletal network was found to have no effect on exocytosis or on general vesicle mobility (Job and Lagnado, 1998;Holt et al, 2004). Vesicles in cone photoreceptors are also more mobile than those of conventional synapses (Rea et al, 2004). At conventional synapses, vesicles are anchored to the actin cytoskeleton via vesicular synapsin, and vesicle mobility is far more restricted.…”

“…In both bipolar cells (Holt et al, 2004) and cone photoreceptors (Rea et al, 2004), diffusion times for cytoplasmic vesicles have been calculated from fluorescence data. The estimated diffusion coefficients (D) (ca.…”

“…Both the speed and the ATP requirement argue against a role for kinesin and myosin motor proteins, including kinesin II (Howard, 1997;Zhang and Hancock, 2004). This movement is also too fast to be explained by simple diffusion, based on the published diffusion coefficients for vesicles in bipolar (Holt et al, 2004) and photoreceptor (Rea et al, 2004). One possible explanation is that vesicles fuse with each other to form compound vesicles prior to fusion with the plasma membrane (Fig.…”